LEUNG SIU-TONG (Tse Tin Wah) joins the police force again to train up undercover cops for the Criminal Intelligence Bureau. He gets to know triad member SO SING-PAK (Bosco Wong) and becomes very good friends with him. PAK is not only smart guy also calculating. He agrees to work as TONG’s informer only because he sees TONG as a stepping-stone to the role of gang leader.

His girlfriend Yiu Ho Ho (Kate Tsui), is a barrister. Despite the fact that they have known each other since childhood, they have never really trusted each other. Tong's supervisor, Inspector Chow Mong Ching (Fala Chen), looks alike his former girlfriend. Having spent a lot of time together at work, the partners at work have finally turned lovers, but their relationship is always full of conflicts and tensions.

Ching believes Tong will not like disciplined work. Her teacher, superintendant Kung Ka Pui (Damian Lau) shares her view later, as Pak's ambition grows, he goes so far as to sacrifice Tong's teammate. With a view to avenging for his mate, Tong decides to return to his life as an undercover agent...